This invention relates to cylinder-pressure based engine control using the methodology referred to as pressure-ratio management. In the context of this invention, the term xe2x80x9cpressure ratioxe2x80x9d refers to the ratio of the fired air-fuel mixture pressure in an engine cylinder at a given crank angle (i.e., piston position) to the pressure of a motored (no ignition) engine when the cylinder contains the same mixture at the same volume. As will be seen, the term xe2x80x9cpressure-ratio managementxe2x80x9d refers to the use of such ratios in a programmed engine control computer to manage certain parameters affecting engine operation. More specifically, this invention relates to the use of pressure-ratio management of an internal combustion engine to detect partial-burn and misfire situations in a cylinder.
Development of advanced engine control systems for the modern four-stroke gasoline engine is being driven by demand for higher fuel economy and increasingly stringent exhaust emission standards. Moreover, the further development of such systems is driven by requirements in the United States, for example, for on-board diagnosis (OBD II) of engine operating events that could adversely affect the catalytic converter or other emissions control equipment.
Individual-cylinder pressure-based feedback is a suitable method to optimize engine operation because engine cylinder pressure is a fundamental combustion variable that can be used to characterize the combustion process for each combustion event. For example, it has been demonstrated that optimal engine control can be maintained by monitoring the pressure in each cylinder and using that information for feedback control of spark timing, exhaust gas recirculation (EGR), air-fuel ratio (A/F), fuel balancing between cylinders, and combustion knock.
Frederic Matekunas has demonstrated (see U.S. Pat. Nos. 4,621,603; 4,622,939 and 4,624,229) that a methodology called xe2x80x9cpressure-ratio managementxe2x80x9d can be used in computer-based, closed-loop, engine-combustion control to better manage air-fuel ratio (including fuel balance between cylinders), ignition timing and EGR dilution, respectively. The teachings of these three patents are incorporated herein by reference. Matekunas"" pressure-ratio management (PRM) involves computer-based engine controls and control algorithms which are facilitated by the availability of a production-viable, reliable cylinder-pressure sensor. The PRM methods require only a signal that has a linear relationship to the cylinder pressure without knowledge of either the gain or the offset of the cylinder pressure related signal. This provides the potential of applying sensors which need not be calibrated and which may measure pressure by means which are less direct than those sensors which must be exposed to the combustion gases in the engine cylinder. Such a sensor is a non-intrusive device called the xe2x80x9cspark-plug bossxe2x80x9d cylinder-pressure sensor as disclosed in U.S. Pat. No 4,969,352 to Mark Sellnau. Some features of PRM will be summarized here because they can be used in combination with the processes of this invention.
PRM uses pressure data from one or more individual engine cylinders, at specified piston positions and corresponding known cylinder volumes. The data is used in the form of the ratio of the fired cylinder pressure and the xe2x80x9cmotored pressurexe2x80x9d (i.e., the pressure that would exist in the cylinder due to the presence of an air and fuel mixture if combustion did not occur). Pressure ratio is calculated for a piston position in terms of the current crank angle position, xcex8, in accordance with the following equation 1.
xe2x80x83PR(xcex8)=P(xcex8)/Pmot(xcex8)xe2x80x83xe2x80x83(1)
Plots of fired pressure and motored pressure data for a cylinder over a range of crank angle positions before and after the top dead center position of the piston are shown in FIG. 1A. FIG. 1B is a graph of pressure ratios (PR) corresponding to the pressure data of FIG. 1A. As seen in FIG. 1B, the PR has unity value before combustion and rises during combustion to a final pressure ratio (FPR) which depends on the amount of heat release per unit charge mass of combustible fuel and air mixture.
The increase in the final pressure ratio is called the modified pressure ratio, MPR.
MPR=FPRxe2x88x921xe2x80x83xe2x80x83(2)
The fractional rise in the pressure ratio is an estimate of the mass burn fraction in the cylinder during a single combustion event. As described in the Matekunas patents, the accuracy of the estimate is influenced only slightly by heat transfer and piston motion. Since the pressure ratio is, by definition, a ratiometric measure of cylinder pressures, PRM algorithms do not require the gain of the pressure sensor. The bias of the pressure signal (voltage) is computed using two compression samples with the assumption of polytropic behavior, which is satisfied whether or not the signal is an absolute pressure. Therefore, PRM is inherently insensitive to many of the common errors in pressure measurement. Importantly, this enables use of low-cost pressure sensors for practical implementations of the system.
Implementation of PRM (for an uncalibrated and arbitrarily biased pressure signal) requires signal sampling at a minimum of four discrete crank angle locations for which cylinder volume is known. [With an absolute pressure transducer, only a single early sample point is required. The mechanization described here is based on triggering of the pressure samples using a 24-tooth crank wheel, which provides the ability to sample at 15-degree intervals. The wheel is aligned to provide a sample at 10 degrees (ATDC).] Suitably, two samples are taken prior to significant heat release, typically 35 and 50 crank angle degrees before the top-dead-center (BTDC) position of the piston on the compression stroke, for determination of the motoring-pressure waveform (see FIG. 1A) and the pressure-sensor signal bias, both from polytropic relationships. A sample taken after combustion is complete, typically at the piston position characterized by 55 crank angle degrees after top dead center (ATDC), is needed to determine the FPR, which is represented as quantity B in FIG. 1B. A sample taken at 10 crank angle degrees ATDC (during combustion) provides the pressure ratio at this sampling point. The fractional rise in pressure ratio is an estimate of the mass-burn fraction in the cylinder in which the pressure is measured. For the 10 degree ATDC point, this is represented by the quantity A/B in FIG. 1B and is also used by Matekunas as a PRM combustion timing parameter, referred to as PRM10.
PRM10=[PR(10)xe2x88x921]/[FPRxe2x88x921]xe2x80x83xe2x80x83(3)
The PRM10 timing parameter (equation 3) is a very sensitive measure of combustion phasing and is useful for minimum ignition (spark) advance for best torque (MBT) spark timing control.
PRM10 values range between 0 and 1. For spark-ignited engines, MBT spark timing usually yields a PRM10 value of about 0.55 with only slight sensitivity to mixture strength and engine speed. As shown in FIG. 1B, an exemplary value of MBT timing is spark ignition at 40 degrees BTDC. Retarded spark timing yields lower values of PRM10; advanced timing yields higher values of PRM10. Typically, a change of 0.1 in PRM10 corresponds to 3 to 5 crank angle degrees change in spark timing. Because the mass burn rate and the slope of the PR curve are near their maximum at 10 degrees ATDC (e.g., see FIG. 1B), the PRM10 parameter remains a sensitive measure of combustion phasing even for high dilution ratios.
For combustion with MBT spark timing, the value of FPR is a maximum for stoichiometric mixtures with no dilution, and decreases as excess air, EGR, or residuals are increased. Therefore, FPR is useful as an indicator of total charge dilution, and is applicable to the control of mixture dilution in systems which are lean burn, use high amounts of EGR, or vary the amount of residual through variable valve train systems. For spark-ignited engines with MBT spark timing, FPR has a typical range between 2.8 and 4.0.
FPR=PR(55)xe2x80x83xe2x80x83(4)
The FPR value varies with cycle timing, higher for retard from best timing and lower for advance. As discussed in the original PRM patents, the correlation of FPR with cycle timing as measured by PRM10 allows the calculation of the expected value at best timing based on the amount of retard or over advance from best timing. This allows for an individual cycle estimation of the mixture based on FPR(MBT) even though the timing of the cycle is not at best timing. The FPR(MBT)xe2x88x921.0 calculated using this correlation is called the Dilution Parameter, or DILPAR. (Subtraction of 1.0 from the FPR value provides a parameter that directly proportions with the ratio of fuel burned to total mass of dilution.)
DILPAR=FPR+(0.5*PRM10)xe2x88x921.275xe2x80x83xe2x80x83(5)
DILPAR provides an estimate of total charge dilution for any one cycle that burns completely. DILPAR exhibits lower cyclic variability than FPR (or MPR), which include the effects of combustion phasing. Since the total charge dilution that a combustion system can tolerate remains nearly constant over the full operating range, DILPAR is a very useful estimator for combustion control with lean A/F ratios or high EGR.
From this understanding of the Matekunas pressure ratio and its use to characterize the combustion process, a variety of engine diagnostic and control strategies have been conceived. The overall engine control strategy for this system was to deliver EGR near the dilution limit, optimize individual-cylinder spark timing, and balance A/F ratio between cylinders, adaptively, for maximum fuel economy and minimum emissions over the life of each vehicle.
While the teachings of the above-identified Matekunas patents provide the basis for substantial improvements in closed loop engine controls, there remains a need for further improvements with respect to detection of partial burns of the combustible mixture in a cylinder, the detection of misfires, and spark-retard control under various engine conditions. Reliable misfire detection is necessary for the U.S. Federal OBDII diagnostic requirements for protection of the vehicle""s catalytic converter over the full range of engine operating speed and load. However, partial burn situations can also contribute to catalyst degradation, and they often arise before a misfire occurs. Furthermore, accurate and reliable partial burn detection would enable more efficient use of engine control strategies in engine cold starts, and during transmission gearshifts and periods of engine idle. For such conditions, high levels of retard can introduce very late burning or partially burning cycles intentionally which might erroneously be considered xe2x80x9cmisfire cyclesxe2x80x9d by speed variation or ion sense misfire detection schemes.
Accordingly, it is an objective of this invention to provide a process using PRM that increases its capability so that it can be used to reliably sense partial burns and misfires on a cylinder by cylinder basis. It is a further objective of this invention to provide such a process that is robust when the combustion event is severely retarded and to provide spark-timing control under these conditions.
In a first embodiment, this invention is a process that uses cylinder pressure ratio (as defined in equation 1), evaluated at a suitable crank angle during the expansion stroke in an internal combustion engine as a measure of the fraction of the fuel burned. This valuation enables calculation of the unburned fuel entering the exhaust for each cylinder and each engine cycle. Expressed as a series of equations, this embodiment of the invention is a process comprising the following calculation steps:
Fraction of Fuel Burned=[PR(crank angle 1)xe2x88x921.0]/[PR(complete burn)xe2x88x921.0]
Fraction of Fuel Unburned=1.0xe2x88x92Fraction of Fuel Burned
Unburned Fuel/Cycle/Cylinder=Fraction of Fuel Unburned*Fuel Delivered per Cycle
PR(Crank Angle 1) is the pressure ratio evaluated at the selected crank angle. An example of a suitable Crank Angle 1 is 55 crank angle degrees ATDC.
The PR (complete burn) is the value of the pressure ratio had combustion gone to completion at crank angle 1, e.g., a FPR. This value can be determined for the particular operating condition through experiment or estimated based on the relative dilution of the charge by residual combustion gasses, excess air, or EGR for the operating condition. For weak mixtures corresponding to high levels of charge dilution [by residuals (light load), excess air (lean) or EGR], the PR for complete burning corresponds to a number close to 3.0 for conventional gasoline engines. For engines capable of very lean operation (stratified charge or diesel engines), the PR for complete burning will be lower and will scale approximately by the relation:
PR(complete burn)xe2x88x921=constant*mass fuel/(mass air+mass EGR+mass fuel)
The PR for complete burning for operating conditions with relatively low charge dilution (high load, no EGR, stoichiometric mixtures) is somewhat over 4.0.
The cumulative amount of fuel entering the exhaust for a given period of time from all cylinders is computed by summing the unburned fuel estimates for individual cycles and individual cylinders for a period of time or number of cycles. This amount is compared to a critical value that would result in catalyst damage, as established through testing or calibration, for the particular engine system and for a range of operating conditions. (For example, during the engine test, a thermocouple could be located in the catalyst to determine if critical temperatures were reached and catalyst damage could occur).
Should the unburned fuel flow rate exceed the critical value, two actions could result. A diagnosis of the engine-operating condition is performed and appropriate xe2x80x9ccorrective actionxe2x80x9d is taken, such as advancing the spark timing, to reduce the unburned fuel rate and/or protect the catalyst. Depending on whether vehicle engine controller provides individual cylinder control, the correction is made to individual cylinders or to a group of cylinders on an averaged basis. If corrective action for the condition is unsuccessful or unacceptable, the driver is notified via a light on the dashboard. The objective of this unburned fuel calculation process is to prevent catalyst damage in as many cases as possible.
The invention may also be practiced in accordance with a related embodiment. Instead of using a selected pressure ratio to calculate the mass of unburned fuel leaving a cylinder or group of cylinders, a modified pressure ratio (equation 2) using a suitable final pressure ratio may be selected that characterizes complete combustion, a partial burn or a misfire. The final pressure ratio is calculated using a fired pressure taken at a crank angle when combustion should have occurred. Under these circumstances, a MPR close to zero indicates a misfire on the tested cycle and a value less than about two may be indicative of a partial burn. If such a low MPR is detected for a cylinder, the test is repeated for a few cycles to confirm that current engine operation parameters, or ignition failure, are causing combustion problems. As described in the above embodiment, engine control corrective action, such as spark advance, is undertaken and/or a notice is given to the vehicle operator of the possible damage to the catalytic converter.
Other objects and advantages of the invention will become apparent from a detailed description of preferred embodiments of the invention that follows below.